Pressure-sensitive labels have become almost ubiquitous in modern society. As only one example, they appear on the vast majority of all shipped packages. They are also used to identify parts in manufactured goods and manufacturing equipment. Another example of a very large pressure-sensitive xe2x80x9clabelxe2x80x9d is shelf or drawer liner sold as xe2x80x9cContac(copyright)xe2x80x9d paper.
As another instance of the use of pressure-sensitive labels, baggage tags (or more commonly xe2x80x9cbag tagsxe2x80x9d), have become unavoidable, certainly in the vicinity of commercial airports. The airlines affix the tags to passengers"" luggage to indicate both the destination of the articles as well as the items"" owners. To include this information, airlines actually print tags when a passenger appears for a flight. The attendant then takes the printed tag, removes a liner sheet, slips it through the handle of the luggage, and adheres it to itself. If the parcel has no handle, such as a carton, then the tag simply sticks directly to the item.
The construction of a typical pressure-sensitive label currently in use appears in FIG. 1. There the label, indicated generally at 11, includes the face sheet 12 upon which appears the desired printing 13. In the case of a shipped package, the information identifies and locates the shipper and the recipient. For the example of bag tags, the printing may not only identify the bag""s destination and owner, but will usually indicate the airline on which the passenger commences his or her journey. The airline identifier (and logo) typically arrives preprinted on the tag when delivered to the carrier by the tag""s producer.
The face sheet generally has a composition of paper or plastic. The information 13 for a general label can be placed on it by a computer""s printer or any suitable device.
For a bag tag, the identification of the bag""s destination and owner can only be placed on the tag 11 at the airport. To accomplish this printing, many if not most, baggage tags"" face sheet 12 have a thermally printable composition. Suitable thermally-printable papers are provided by Appleton Papers Inc. of Appleton, Wis., and by Ricoh Electronics Inc. of Santa Ana, Calif. Each of the baggage stands then possesses a thermal printer which, through the application of heat, places the necessary information on the tag""s face sheet 12.
Accordingly, the process described below will produce label stock for a very wide range of pressure sensitive labels. Most labels may utilize ordinary paper or plastic as the face sheet 12. Bag tags can employ the direct thermal printing process described above.
In addition, the indirect thermal printing process, as opposed to the direct thermal, process described below, uses ordinary, but smooth, paper as the face sheet 12. The thermal printing head actually applies the heat to a ribbon which contains ink. The heat releases the ink from the ribbon which deposits it on the face sheet to produce the printing.
With the information 13 on the face sheet 12, the operator removes the liner sheet 15 from the adhesive 16 on the backside of the face sheet 12. With the adhesive 16 exposed, the user places the face sheet 12 with the adhesive directly on the package or the equipment. Dyna-Tech Adhesives Incorporated of Grafton, W. Va. provides a suitable general-purpose adhesive under the designation DyTac 2057. This adhesive has a composition of a styrene-butadiene polymer and constitutes an organic phase dispersed as an emulsion in an aqueous phase.
For a bag tag, the airline personnel places part of the tag through the bag""s handle and sticks the adhesive to a portion of the tag which still the liner sheet attached. Sticking the adhesive to the remaining liner forms a loop of the tag around the handle. Alternately, if the bag or parcel has no handle, the attendant removes all of the liner 15 from the tag 11 and sticks it directly to the item.
In either event, the liner must remove virtually completely from the adhesive so that the latter can facilely attach to another surface. If portions of the liner 15, typically composed of paper, remains on the adhesive 16, then the covered adhesive would not achieve a secure affixation of the label to its item. In the case of bag tags, the resulting dislodged tag would create a lost bag to the distress of both the passenger and the airline.
To achieve a clean separation of the liner 15 from the adhesive 16, the former carries the silicone coating 19 attached to it. The coating 19, firmly affixed to the liner 15, presents a smooth, relatively nonadherent surface to the adhesive 16. Pulling off the liner 15 with its silicone coating 19 results in a clean, sticky layer of adhesive 16 attached to the face sheet 12. Stated in other words, the silicone coating 10, affixed to the liner 15, permits the release of the adhesive 16 from itself and thus the liner 15. This leaves the face sheet 12 with an exposed layer of adhesive 16.
The manufacture of the label 11 generally commences with the coating of the paper liner 15 with the silicone 19. A type of silicone finding frequent use in this day of environmental concerns takes the form of a UV curable silicone polymerizable liquid. Use of this category of silicone avoids the problem of evaporating hydrocarbons into the atmosphere. Curing the liquid silicone entail shining it with UV radiation. This causes the silicone monomers to combine, leaving a unified mass of silicone coating 19 firmly attached to the liner 15. Blissfully, the process places substantially no pollutants into the atmosphere.
Suitable components for UV-curable silicone solutions are supplied by Rhodia Inc. of Rock Hill, S.C. A liquid for an easily releasable layer may include 100 parts of UV-curable silicone polymer (Rhodia product PC-600); 2.5 parts of cationic photoinitiator (PC-702); and CRA-minus release modifier (PC-670). The silicone itself has a composition of an epoxy-modified polydimethylsiloxane. This type of compound comes from the class of epoxy-modified polysiloxanes which may well provide other suitable moieties.
For a silicone layer with a tighter release, the above solution may include 5 to 55 parts (depending upon the desired characteristics of the silicone layer) of CRA-plus release modifier (PC-680). Free radical photoinitiators may alternately find use in appropriate solutions.
Promptly after receiving the UV radiation, the silicone 19 cures into a reasonably hard mass firmly affixed to the paper liner 15. However, it cannot then receive the adhesive 16, Placing the adhesive onto the newly cured silicone layer 19 will cause these two layers 16 and 19 to stick together. This represents a clearly unacceptable situation. Instead, to avoid this clearly deleterious situation, the silicone layer 19 must sit on the liner 15 at least several hours if not a whole day before contacting the adhesive 16 to avoid the unacceptable results indicated above. The necessity for delay seems remarkable in light of the fact that the newly cured silicone seems hard, smooth, and ready for further use. In fact, during the delay, the silicone-coated liner can be wrapped into a large roll and, after the delay period, unrolled and used without displaying any negative effects.
In fact, rolling the liner with the newly cured silicone into a large cylinder represents the usual manufacturing process. Simply stated, the liner with the new silicone coating cannot remain on the manufacturing equipment and render it useless for the time required for the silicone to remain out of contact with an adhesive. Accordingly, the usual manufacturing process involves a machine placing the uncured silicone on the liner 15. The silicone undergoes UV curing. Then the machine rolls the liner 15 with the cured silicone layer 19 onto a roll, which is removed from the machinery and left to sit for the substantial period of time necessary to render the silicone substantially less adherent to the adhesive 16. After the delay period, the roll is then placed onto machinery. This permits the completion of the manufacturing process by placing adhesive 16 onto the silicone 19 and then attaching the face sheet 12.
Removing the roll of silicone-coated liner, allowing it to simply stand for an extended period of time, and placing it back on a machine again represents a substantial manufacturing effort. Eliminating these steps would achieve a significant economy in the production of pressure-sensitive label stock.
Heating the silicone coating after its UV curing and waiting briefly before applying an adhesive permits the further processing of the material without the necessity of removing it from the equipment and allowing it to stand idle. Although the silicone appears cured and hard, the subsequent heating and short wait places it in condition to contact the adhesive without excessively sticking to it.
Fundamentally, an improved method of treating a UV-cured silicone layer on a substrate comprises first heating the UV-cured silicone at an elevated temperature. After heating the silicone layer at the elevated temperature, the silicone layer is kept free from contact with an adhesive for a predetermined period of time. This time period conditions the heated silicone to receive the adhesive without deleterious effects and depends primarily on the silicone and the adhesive.
After the passage of the predetermined period of time after heating the silicone at the elevated temperature, an adhesive layer may be applied to the silicone. This process permits the subsequent separation of the two layers when they constitute part of a label.
Completing the pressure-sensitive label stock involves adhering a face sheet to the adhesive. The face sheet, of course, will carry the information identifying the equipment, the package""s sender and recipient, or the bag""s destination and owner. Actually, the face sheet may adhere to the adhesive either before the adhesive is applied to the liner or after. If before, the adhesive forms a layer on the face sheet and the two placed against the liner with the silicone coating, with the adhesive contacting the silicone.
Actually, the complete process of making the label stock starts with coating a liner sheet, usually paper, with an uncured, liquid, UV-curable silicone. The film of liquid silicone undergoes curing through exposure to UV radiation. Finishing the bag tags then proceeds through the various steps outlined above. With the heating of the cured silicone followed by the short delay, the materials need never leave the manufacturing equipment from the beginning to the end.